DESCRIPTION (taken from the application) The proposed studies will augment the specific aims proposed by the P.I.'s K08 grant (DK02365-01) and represent a new area of research for the P.I., and will extend her fund of knowledge in tracer methodology. Patients with cystic fibrosis (CF) have a high incidence of abnormal glucose tolerance. Our group has recently described the contribution of elevated hepatic glucose production (HGP) and hepatic insulin resistance to abnormal glucose tolerance. Both gluconeogenesis and glycogenolysis contribute to HGP; however, their relative contribution to HGP in CF has not been determined. We recently reported significantly lower rates of glucose oxidation (glycogen storage) during hyperinsulinemia in CF subjects. If glycogen storage is diminished in CF, gIuconeogenesis may play a more important role in early post-absorptive HGP. Thus the contribution of gluconeogenesis and glycogenolysis to hepatic glucose production is important. The first major specific aim of this study is to utilize tracer methodology to determine the relative contribution of gluconeogenesis and glycogenolysis to hepatic glucose production in CF. Cystic fibrosis is a catabolic condition, yet the etiology of catabolism is poorly understood. Using indirect calorimetry, we recently reported that CF patients have de novo lipogenesis. Significant de novo lipogenesis could potentially be energy wasteful and could contribute to high resting energy expenditure in CF. A second major specific aim of this study is to utilized tracer methodology to determine the contribution of de novo lipogenesis to resting energy expenditure in CF. We will recruit 24 adult CF subjects and compare results to those from 10 control volunteers matched for lean tissue mass, is well as age, weight and sex. Additionally, all subjects will be characterized by a three hour oral glucose tolerance test. All normal volunteers will have normal glucose tolerance. We will subgroup CF subjects based on glucose tolerance (normal glucose tolerance, impaired glucose tolerance, or diabetes, eight per subgroup). We will determine descriptive statistics of gluconeogenesis and de novo lipogenesis for each CF subgroup as pilot data for future grant submission. Using the stable isotope deuterium (in D2O), we will measure incorporation of 2H2 into the second and sixth carbon label of glucose, to quantify gluconeogenesis according the method of Landau. We will utilize the isotopomer equilibration method, as developed by Hellerstein and modified by Aarsland, to measure de novo lipogenesis using the stable isotopes [ 1,2-13C] acetate and [1-13C] palmitate. We will utilize indirect calorimetry to measure resting energy expenditure (REE) and substrate utilization. We will correlate REE with de novo lipogenesis and we will compare measurements of lipogenesis obtained from calorimetry to direct measurements obtained using stable isotopes. These methods will test our two major hypotheses: 1) In the post-absorptive state, gluconeogenesis contributes to a larger percentage of HGP in CF than in controls; 2) de novo lipogenesis is a significant metabolic pathway in CF and correlates with REE.